1. Field of the Invention
The present invention relates to an optical pickup for recording and/or reproducing a signal on and/or from an optical disk such as a minidisk (MD), a magneto optical disk (MO), a compact disk (CD) and a CD-ROM, and further to an optical disk unit equipped with this optical pick up.
2. Description of the Related Art
So far, as an optical pickup for an optical disk, various types using a hologram device have been proposed for the purpose of simplifying, stabilizing and size-reducing an optical system, for example, constructed as shown in FIG. 3.
In FIG. 3, an optical pickup, generally designated at numeral 1, is made up of a light-emitting device 2, a hologram device 3, an objective lens 4 and an optical detector 5.
The hologram device 3 has two surfaces perpendicular to the optical axis of a light beam from the light-emitting device 2, with a diffraction grating 3a being formed in the first surface (lower surface in FIG. 3) existing on the light-emitting device 2 side and on the optical axis of the light beam from the light-emitting element 2.
In addition, the hologram device 3 has a hologram 3b formed in its second surface (upper surface in FIG. 3) existing on an optical disk D side and on the optical axis of the light beam from the light-emitting device 2.
The hologram 3b allows the light beam from the light-emitting device 2 to directly pass, and makes the light returning from the optical disk D to diffract to be led into the aforesaid optical detector 5.
In this instance, the hologram 3b is, as illustrated, composed of two hologram sections 3b-1, 3b-2 made by division along a direction parallel to a track direction of the optical disk D, and the hologram sections 3b-1, 3b-2 have discontinuous spatial frequencies with respect to each other.
In the case of being illustrated, the optical detector 5 comprises a central light-receiving section 5a for receiving a main beam after the division by the diffraction grating 3a and light-receiving portions E, F provided on both sides of the light-receiving section 5a to receive side beams, and further, the light-receiving section 5a is composed of four light-receiving portions A, B, C and D established by vertically and horizontally dividing the light-receiving section 5a into four.
Furthermore, in a non-shown processing circuit, detection signals from the aforesaid light-receiving portions A, B, C, D, E and F are amplified by a head amplifier into output signals Sa, Sb, Sc, Sd, Se and Sf.
In addition, in an arithmetic circuit, for instance, a reproduction signal RF1 is calculated according to the following equation (1). EQU RF1=(Sa+Sb)+(Sc+Sd) (1)
Further, a focus error signal FE1 is given according to the following equation (2) or (3). EQU FE1=(Sa-Sb) (2) EQU FE1=(Sa+Sd)-(Sc+Sb) (3)
Still further, a tracking error signal TE1 is given by the following equation (4). EQU TE1=(Se-Sf) (4)
In the optical pickup 1 thus constructed, a light beam from the light-emitting device 2 is divided through the diffraction grating 3a of the hologram device 3 into a main beam and two side beams, and subsequently, passes through the hologram 3b to reach a signal recorded surface of the optical disk D by means of the objective lens 4.
A returning optical beam reflected on this signal recorded surface again passes through the objective lens 4 to be incident on the hologram 3b, where this returning light is diffracted by each of the hologram portions 3b-1, 3b-2 of the hologram 3b so that the returning light due to the main beam is incident on the light-receiving section 5a of the optical detector 5 while the returning light resulting from the side beams are incident on the light-receiving portions E, F of the optical detector 5.
Whereupon, on the basis of the detection signals from the respective light-receiving portions A, B, C, D, E and F of the optical detector 5, the detection of the reproduction signal RF1, the focus error signal FE1 and tracking error signal TE1 are possible.
In addition, there has also been known an optical pickup shown in FIG. 4.
In FIG. 4, an optical pickup, generally designated at numeral 6, includes a light-emitting/receiving device 7, a hologram device 3 and an objective lens 4.
The light-emitting/receiving device 7 is, as illustrated, constructed integrally on one semiconductor substrate 7a.
More specifically, in the light-emitting/receiving device 7, a light-emitting section is made up of a semiconductor laser element 7b formed on a semiconductor substrate 7a and a reflecting mirror 7c formed to be inclined obliquely from a surface of the semiconductor substrate 7a. In addition, an optical detector is composed of first and second optical detectors 8, 9 placed on the semiconductor substrate 7a.
In this case, the aforesaid hologram device 3 has two surfaces perpendicular to the optical axis of a light beam from the light-emitting/receiving device 7, and a diffraction grating 3a is formed in the first surface (lower surface in FIG. 4) existing on the light-emitting/receiving device 7 side and on the optical axis of the light beam from light-emitting/receiving device 7 while a hologram 3c is formed in the second surface (upper surface in FIG. 4) existing on an optical disk D side and on the optical axis of the light beam from light-emitting/receiving device 7.
In this instance, the aforesaid hologram 3c is constructed as being one continuous area, and allows the light beam from the light-emitting/receiving device 7 to directly pass and diffracts the returning light from the optical disk D so that 1 order light and -1 order light are led into the first and second optical detectors 8, 9, respectively.
As illustrated, each of the optical detectors 8, 9 comprises a central light-receiving section on which a main beam divided by the diffraction grating 3a is incident and light-receiving sections 8e, 8f or 9e, 9f located on both sides of this central light-receiving section for receiving side beams, with the central section having light-receiving portions 8a, 8b and 8c or 9a, 9b and 9c established by dividing the central section into three along division lines parallel to the track direction of the optical disk D.
Furthermore, in a non-shown processing circuit, the detection signals from the light-receiving portions 8a, 8b, 8c, 8e and 8f or 9a, 9b, 9c, 9e and 9f are amplified by a head amplifier into output signals SA, SB, SC, SE, SF, SG, SH, SI, SK and SL. In addition, for example, a reproduction signal RF2 is given according to the following equation (5). EQU RF2=(SA+SB+SC)+(SG+SH+SI) (5)
Further, a focus error signal FE2 is given by the following equation (6). EQU FE2=(SA+SH+SC)-(SG+SB+SI) (6)
Still further, a tracking error signal TE2 is given by the following equation. EQU TE2=(SE+SK)-(SF+SL) (7)
In the optical pickup 6 thus constructed, a light beam from the light-emitting/receiving device 7 is divided into a main beam and two side beams, and subsequently, passes through the hologram 3c to reach a signal recorded surface of the optical disk D through the objective lens 4.
The returning light beam from this signal recorded surface is incident on the hologram 3c after again passing through the objective lens 4. In this instance, the returning light is diffracted by the hologram 3c so that 1 order light and -1 order light are incident on the first and second optical detectors 8, 9, respectively.
Whereupon, on the basis of the detection signals of the light-receiving portions of the optical detectors 8, 9, the detection of the reproduction signal RF2, the focus error signal FE2 and the tracking error signal TE2 are possible.
However, the optical pickups 1, 6 constructed as described above produce the following problems.
First, in the case of the optical pickup 1, although the focus error signal is made to be detected according to the so-called Foucault's method, in order to lead the returning light due to the main beam into each of the detecting portions A, B, C and D of the optical detector 5, there is a need for the hologram 3b of the hologram device 3 to be constructed to include a plurality of areas 3b-1, 3b-2 having different diffraction angles.
Accordingly, a mask metal pattern or the like for manufacturing the hologram device 3 through the use of the etching, injection molding or the like becomes complicated, which raises the manufacturing cost.
On the other hand, in the case of the optical pickup 6, the focus error signal is designed to be detected according to the so-called beam size method, and the hologram 3c of the hologram device 3 is constructed as a continuous area, and therefore, as compared with the optical pickup 1, the hologram device 3 is easily producible at a lower cost.
However, in this case, from the principle of the beam size method, there is a need to enlarge the diameter of a spot of the returning light on the optical detectors 8, 9 of the light-emitting/receiving device 7. For this reason, the distance between the light-receiving portions 8e, 8f or 9e, 9f for the detection of the tracking error signal and the central light-receiving section is prolonged, and hence, difficulty is experienced in considerably shortening the distances of the main beam and the side beams on the signal recorded surface of the optical disk D, and the tracking error signal tends to easily receive the influence from cuts or the like on the signal recorded surface of the optical disk D.
Moreover, due to the feature of this focus error detecting method, a false focusing signal takes place in an area in which the focus error is large as compared with the depth of focus of the objective lens, which requires a countermeasure against this false focusing signal.